The present invention relates to preservative coatings and particularly to an improved water based, quick-drying, anti-corrosive paint.
The atmosphere surrounding some industrial plants such as pulp and paper mills and chemical production plants and the atmosphere along a sea coast present difficult problems of preservation of materials exposed to the weather. Steel objects such as bridges and ship hulls need a protective coating to prevent rapid surface deterioration in such atmospheric conditions.
Protection of the surface of ferrous materials in a corrosive atmosphere is particularly difficult, since rust on such surfaces can progress beneath a coating of ordinary paint. Any break through a coating of ordinary paint may permit corrosion to begin, and such corrosion will ordinarily spread from such a break, under the paint. It is therefore important for a paint to be strongly adhesive and to dry quickly to a strong impact resistant coat.
A previously known paint which provides excellent protection for metal surfaces exposed to a corrosive atmosphere includes a pigment which comprises finely flaked stainless steel, finely ground mica flakes, and barium sulfate, with a binder comprising a combination of acrylic resins in aqueous emulsion. While this paint has proved to be very durable, even in the severely corrosive atmosphere surrounding pulp and paper plants an chlorine plants, it suffers from a lack of resistance to flash rusting. That is, when the previously known paint was applied to a rusty ferrous metal surface a rust colored stain often appeared on the surface of the paint coating, detracting from its appearance.
The occurrence of such flash rusting is thought to be a result of iron ion migration through the paint film, perhaps enhanced by the chemical agents used to emulsify the acrylic resins included in the previously known paint. Although flash rust stains may be physically removed from the surface of the paint, the stains and their removal detract from the appearance of the paint surface.
In order to be highly resistant to physical and chemical damage, water-based paints have previously been manufactured in which large amounts of resin binders and pigment material are used in relation to the amount of water. Properly mixing such paint results in large amounts of air becoming entrapped in the paint in the form of small bubbles. As a result, each batch of such paint has to be allowed to rest for periods as long as two days before it can be accurately measured into containers for marketing.
What is desired, then, is an improved paint for use in exceptionally corrosive environments, which is not subject to flash rusting, which dries rapidly, and which may be packaged without long delay after its preparation.